Participation in flexible contract work has increased dramatically over the last decade, often in settings where new technologies lower the transaction costs of providing labor flexibly. One prominent example of this is the ride-sharing company Uber, which allows drivers to provide (or not provide) rides anytime they are willing to accept prevailing prices for this service. An Uber-style arrangement offers workers flexibility in both setting a customized work schedule and also adjusting it throughout the day. Using high-frequency data of hourly earnings for Uber drivers, we document the ways in which drivers utilize this real-time flexibility and we estimate the driver surplus generated by this flexibility. We estimate how drivers' reservation wages vary in high frequency from hour to hour, which allows us to study the surplus and supply implications of both flexible and traditional work arrangements. Our results indicate that, while the Uber relationship may have other drawbacks, Uber drivers benefit significantly from real-time flexibility, earning more than twice the surplus they would in less flexible arrangements. If required to supply labor inflexibly at prevailing wages, they would also reduce the hours they supply by more than two-thirds. The implications of our findings for the future of flexible work are discussed.